The invention relates to an illumination system having an active layer which comprises an electroluminescent material and is present between an optically transparent electrode layer and a reflective electrode layer.
The invention also relates to a flat-panel picture display device comprising such an illumination system.
An illumination system of the type described in the opening paragraph is known from, for example, the article: "Polymer LEDs" by R. Friend, D. Bradley and A. Holmes in Physics World, November 1992, pp. 42-46. The illumination system described in this article comprises a semiconductor polymer film as an active layer which is formed on a first electrode layer which, in its turn, is provided on a glass substrate. A second electrode layer is provided on the polymer film. One of the two electrode layers should be optically transparent so as to pass light in a direction perpendicular to the plane of the illumination system. If a voltage is applied across the two electrode layers, the active layer emits light in accordance with the process of electroluminescence. The active layer and the two electrode layers jointly constitute a planar light-emitting diode (LED). Such an illumination system is particularly suitable for use as, for example, background lighting for a picture display device having a liquid crystalline picture display panel.
In a large number of flat-panel picture display devices comprising a liquid crystalline picture display panel, for generating the picture to be displayed the state of polarization of the light incident on the picture display panel is modulated in conformity with the picture information. To this end, the light incident on the picture display panel is preferably polarized in advance, which means that the illumination system must supply polarized light. To be able to realize this, for example, a dichroic polarizer may be arranged on the surface of the illumination system facing the picture display panel. A drawback is, however, that approximately 50% of the light supplied by the illumination system is absorbed by this polarizer and, consequently, will not contribute to the light output of the illumination system.
Another possibility of having the illumination system supply polarized light is to ensure that the active layer of the planar LED emits polarized light itself. An example of such a LED is described in the international Patent Application WO 92/16023. A drawback is the relatively low polarization efficiency, so that a considerable part of the emitted light does not have the suitable state of polarization to be modulated, so that it will not contribute to the formation of the picture.